1. Field of the Invention
The invention relates to methods for vertical seismic profiling (VSP) of earth formations in response to acoustic waves from a source in the earth, particularly an operating drill bit, and to methods for conducting such vertical seismic profiling without interrupting the drilling operation.
2. Description of the Prior Art
Vertical seismic profiling is usually done by pulling the drill string out of the drilling hole, then lowering a seismic listening device (geophone) into the hole, and listening to or recording the acoustic energy transmitted from a seismic source at the surface. This operation involves a stop or break in drilling activity and thus, a significant increase of the cost of the drilling operation in addition to the cost of the actual profiling operation.
In order to simplify profiling techniques, attempts have been made in the past to provide sources of seismic energy, both of the continuous-wave and impulsive kind, in a downhole position. In all these cases, knowledge of the source signature was required for processing the data. The source "signature" is the wavefront expression of the acoustic energy that propagates directly from the source to a receiver, without reflection. Such knowledge of the source signature has, however, been unreliable due to unpredictable coupling of the source through the walls of the well.
Various methods have been proposed to avoid this problem, involving use of arbitrary, unknown, buried sources and surface recording. U.S. Pat. Nos. 2,452,515 and 3,739,871 describe examples of such methods.
According to U.S. Pat. No. 2,452,515, surface recording of acoustic energy generated by the drill bit is used to obtain information on the nature of the material of the formation that is actually being ground by the bit. The method according to that patent, however, provides only such information, and nothing is mentioned in the specification as to how the drill bit may be used as a source of acoustic energy in such a manner that it permits analysis of other areas of the formation, and in particular it does not disclose how to separate acoustic energy from different sources.
According to U.S. Pat. No. 3,739,871, a buried or submersed source is used, and acoustic energy signals are recorded on the surface. The method disclosed in this latter specification will only work with impulsive sources, and the acoustic waves are formed by hydraulic fracturing. It uses an array of geophones on the surface, and the acoustic waves received are used to determine an accurate location of the fracturing and the degree of fracturing. No attempt is made to derive seismic reflection data.
It is an object of the present invention to provide a new and improved method for carrying out vertical seismic profiling to determine the local geology surrounding a drilling well.
It is a further object of the invention to carry out such profiling without any break in drilling operations, and by using the drill bit itself as a source of seismic energy, while providing for this method to be carried out to maximum advantage, both economically and operationally.